The present invention relates to coated ink jet recording sheets and coating compositions used to prepare the same. In particular, the invention relates to coating compositions suitable for preparing glossy ink jet recording sheets which possess good printability characteristics.
Ink jet printing processes are well known. Such systems project ink droplets onto a recording sheet, e.g., paper, at varying densities and speed. When using multi-color ink jet systems, the process projects in very close proximity a number of different colored inks having varying properties and absorption rates. Indeed, these multi-color systems are designed to provide images which simulate photographic imaging, and such images require high resolution and color gamut. Accordingly, ink jet recording sheets must be able to absorb ink at high densities, in a capacity such that the colors deposited are bright and clear, at rates to effect quick drying, absorb ink so that it does not run or blot, and in a manner that results in smooth images.
To meet these goals, highly porous pigments, e.g., porous silicas, have been incorporated into paper coatings. Silica-based coating systems have been successful in meeting the printability goals. However, it has been difficult to obtain such properties and produce a non-matted, or glossy, finish typically seen in traditional photographic systems. The aforementioned porous pigments typically have porosities above 1 cc/g and have average particle sizes greater than 1 micron. Such particle sizes and porosities increase the surface roughness of the finished coating, thereby deflecting incident light so that it is scattered, thereby matting the coating.
To enhance the glossiness of such coatings, second gloss layers are provided on top of ink receptive layers prepared from the aforementioned porous pigments. These top layers are prepared from binder systems that are inherently glossy, or from layers comprising binder and much smaller sized inorganic oxide particles, e.g., conventional colloidal silica. The colloidal silica in the latter approach tends to enhance the ink receptive nature of the top coating, but are not large enough to cause surface deformations. There is, however, a tendency for the colloidal particles to agglomerate at high concentrations, thereby causing imperfections and surface roughness in the top layer, and thereby reducing gloss. Accordingly, lower concentrations (i.e., lower ratios of colloidal solids to binder solids) have been used when employing this approach.
It would therefore be quite desirable to increase the amounts of solid inorganic oxides in these top layers to further improve printability. Indeed, it would be desirable to use coating layers having at least 1:1 colloidal solids to binder solids ratios, and even more preferable to employ coatings having colloidal silica solids to binder ratios as high as 4:1, yet at the same time attain acceptable gloss.
Furthermore, coating systems for ink jet paper are frequently designed to have an overall cationic charge. Many of the inks employed in ink jet processes possess a negative charge; and it, therefore, is desirable for the coating components to have an opposite charge to affix the ink. Colloidal alumina possesses a positive charge and has been widely used as a coating pigment for that purpose. Cationic dye fixing components and cationic binders are also employed. Indeed, the presence of these latter cationically charged materials usually require that the pigment components in the coating be cationic or at least nonionic. Otherwise the materials in the coating tend to aggregate, thereby creating surface imperfections and reducing gloss.
It has been recently discovered that certain cationic colloidal silicas which are deionized can be incorporated into glossy coatings at high levels. However, while such coatings provide good to excellent gloss, they have performance limitations with respect to printability. Certain of the recently developed cationic colloidal silicas also have a relatively short storage life. They either, after several days, become too viscous to be incorporated into a coating resin, or after relatively short periods of time, e.g., a couple of weeks, they are not useable because even though the colloidal silica has not gelled, it results in a matte coating after it is incorporated into the coating mixture. Accordingly, these recently developed colloidal silicas either would be more suitable for use as glossy overcoats placed over a separate ink receptive coating, or they would have to be used shortly after the sols are prepared. It, therefore, would be desirable, and it is a goal of this invention, to provide a coating layer comprising a relatively high content of silica solids which are cationic, yet also provide a coating having good to excellent printability, as well as provide a cationic colloidal silica having longer term storage stability.